The publication "High-T.sub.c Transistors" by J. Mannhart in the Journal of Superconductor Science and Technology, Vol. 9, Nr. 2, page 49 ff, to be published on Feb. 1st 1996 gives an overview over the state of the art concerning so called "Three-terminal superconducting devices". These devices rely on superconductivity in a channel between a source- and a drain-electrode and show electronic properties in the superconducting state that surpass the device-relevant properties of semiconducting devices.
Depicted in the cited publication is a special representative of these devices which is the SUFET (SUpercondcuting Field-Effect Transistor). The SUFET consists of a substrate, a superconductive layer and an insulating layer, the insulating layer serving as gate insulator for a gate-electrode. Further a source- and a drain-electrode are fixed to the superconductive layer. The SUFET-device acts in a similar way as a known Field-Effect-Transistor (FET) which means that a voltage drop over an electrically conductive channel between source and drain can be controlled by a gate-voltage. In this device in particular the value of the Critical-Temperature T.sub.c of the superconductive layer is shifted up or down through the gate-voltage, dependent on the voltage's polarity. In the channel which in this case is superconductive, the distribution of the charge carriers plays an important role. Especially in a superconductor, where the density of the mobile charge carriers is very high in comparison with semiconductors, the effect called electrostatic shielding or screening, which means that the penetration-depth of the gate's electric field into the superconductor is limited because of a strong concentration change of the charge carriers at the superconductor/insulator-interface, causes problems. When electrostatic shielding occurs, the superconducting current in the channel can no longer be effectively controlled by raising the gate-voltage. This effect is delimiting the device's performance.
There have been made efforts to avoid this problem by using ultrathin superconductive layers which however then suffered from severe technological problems. To overcome this drawback in another way, the SUFET was modified to a JOFET (JOsephson Field-Effect Transistor). One type of JOFET consists also of a bicrystalline substrate, a superconductive layer and an insulating layer, the insulating layer serving again as gate insulator for a gate-electrode. In addition the JOFET is provided with a grain boundary protruding through the substrate as well as the superconducting layer and also the insulating layer. The grain boundary serves as a Josephson-junction in the superconductive layer, because there it represents a weak link, disturbing the superconductive behavior of the channel in a predetermined way. At the Josephson-junction the superconductive layer can be interpreted as being interrupted by a small gap of non-superconductive material. The underlying phenomena are the proximity-effect and the tunnel-effect which both have in common that a superconducting current can flow through this gap, when it is narrow enough. In this gap almost no free charge carriers exist and therefore the electrical field from the gate can penetrate much deeper into the gap than into the superconductive layer. The screening-effect is hence suppressed in the direct vicinity of the Josephson-junction. Further also the effective width of the gap can be controlled through the gate-voltage which amplifies the control effect of the gate-voltage. In summary, by the inserted Josephson-junction the negative effect of shielding is bypassed and a better control of the superconducting current is possible.
Due to the manufacturing principle the Josephson-junction in the above mentioned device as well as in the next mentioned device is called a "grain-boundary-induced Grain-boundary-Josephson-junction". In EP 0 329 603 A2 a method is described to produce a bicrystalline substrate with a bicrystalline superconductive layer upon it. This method was also used to manufacture the JOFET, by adding source- and drain-contacts and an insulating layer with a gate-contact on the superconductive layer.
Although this JOFET-device in theory offers an acceptable behavior, in reality a lot of problems occur which deteriorate the device's performance. A main problem is the fact that the superconductive layer which is a very sensitive crystal-layer, is strongly damaged by the gate manufacturing-process. During this process the stoichiometric ratio of the cations in the superconductive layer is deteriorated due to thermal diffusion. Particularly for the superconductive High-T.sub.c material YBa.sub.2 Cu.sub.3 O.sub.7 the cations Y, Ba and Cu are diffusing away. Also a considerable amount of oxygen which is an important element in known high-T.sub.c superconductors, diffuses out of the superconductive layer. This diffusion lowers the Critical-Temperature T.sub.c, making it thereby difficult to achieve a well-working device with an acceptable working-temperature. The gate manufacturing-process also normally uses a plasma for depositing the gate material. The plasma then introduces the drawback that also the superconductive layer's surface is bombarded with ions digging out elements of the superconductive layer and again distorting the stoichiometric ratio. Finally the often used oxygen-atmosphere has a pressure which changes the oxygen-portion of the superconductive layer. All these effects work together to deteriorate the quality of the superconductive layer. An effort is known to bypass these problems by depositing an amorphous insulating layer but this insulating layer showed worse dielectric properties like a low dielectric constant which again lead to very high gate-voltages needed to achieve a reasonable current control. Another problem is the fact that the grain boundary in the gate insulator acts as a leakage path for a gate current which impedes the application of required high gate-voltages. The publication "Field effect transistor based on a bi-crystal grain boundary Josephson-junction" by Ivanov et al. from the Applied Superconductivity Conference, Chicago, Ill., Aug. 23-28, 1992 describes such a JOFET and outlines the difficulties to obtain a viable device.
Efforts on this sort of device with a single grain boundary junction on a bicrystal substrate are also reported in "Electric Field Effect in Sm.sub.l-x Ca.sub.x Ba.sub.2 Cu.sub.3 O.sub.y Bicrystal Junctions" by Dong et al. in IEEE Transactions on applied superconductivity, Vol. 5, No. 2, June 1995, pages 2879-2882, where the effect of overdoping a superconductor is examined and where the junction is treated with photolithography and ion milling. A floating gate is used to protect the superconductive layer while growing the gate but also this device shows a not outstanding superconducting current vs. gate-voltage characteristic.
In "Superconducting thin films for device applications" by D. F. Moore, 2nd Workshop on High Temperature Superconducting Electron devices, R&D Association for Future Electron Devices, Jun. 7-9, 1989, Shikabe, Hokkaido, Japan, pp. 281-284 it is reported that main thoughts are directed to thin film superconductors or to weak links to overcome screening. The described methods show the problems which occur during the manufacturing of the weak links, namely leakage problems or backscattering. To use electron beam exposure for producing the weak links is also mentioned but no results are given for this method.
In addition to the discussed JOFETs with a conventional FET-design another design is known:
From the research report "Large Electric Field Effects in YBa.sub.2 Cu.sub.3 O.sub.7-.delta. Films Containing Weak Links" by Mannhart, Strobel, Bednorz and Gerber in Applied Physics Letters No. 62, pages 630ff, 1992 is known a JOFET-device which comprises a Nb-doped SrTiO.sub.3 -electrically conductive substrate, a non-doped SrTiO.sub.3 -insulating layer and a YBa.sub.2 Cu.sub.3 O.sub.7 -superconductive-layer. The shown JOFET-device has again three terminals, namely a drain-, a source- and a gate-electrode. Whereas the source- and the drain-electrode are fixed as usual on the superconductive layer, the gate-electrode is now fixed on the substrate. The substrate itself is electrically conductive and therefore functions as a part of the gate-electrode. During the manufacturing-process the upper surface of the substrate is treated with a polishing-process whereby fine submicron grooves are produced in the substrate. The grooves serve as sources for the generation of crystal defects in the insulating layer which is manufactured during a following epitaxial process. The crystal defects in the insulating layer are themselves the sources for the generation of crystal defects in the superconductive layer which is produced also during an epitaxial process step. The crystal defects in the superconductive layer serve as weak links which in best cases show Josephson-junction-behavior. These weak links are, due to their manufacturing principle, called "groove-induced Grain-boundary-Josephson-junctions".
The advantage of this special design with the electrically conductive substrate and the gate-electrode on it, called "Inverted JOFET", is the fact that the superconductive layer is not inevitably deteriorated while manufacturing the gate-electrode as it happens with the non-inverted JOFET. However, here the problem is that the Josephson-junctions induced by the grooves show very bad properties, thereby reducing considerably the performance of the device.
In fact the resulting behavior is worse than the behavior of the known non-inverted JOFET. Another problem is the fact that, as a result of the polishing-process, not only one grain boundary but a network of several grain boundaries is produced which comprises more grain boundaries than wanted. Even worse, the grain boundaries are, due to the polishing principle, arranged in an unreproducable pattern. Thereby again no device with an acceptable Critical-Temperature T.sub.c has been achieved and also no acceptable superconducting behavior of the device. Another argument against this type of inverted JOFET-device is the fact that, starting from its design, multiple-gate-JOFETs can only be manufactured with a lot of work.
In "Electric Filed effects on YBa.sub.2 Cu.sub.3 O.sub.7-.delta. Grain Boundary Josephson-junctions" by Nakajima et al. in the Japanese Journal of Applied Physics, Vol. 33 (1994), pp. L934-L937, Part 2, No. 7A, Jul. 1, 1994, a device is described which also uses the inverted design. However an electrically non-conductive substrate-layer is used as the gate insulator. The substrate is bicrystalline to produce the grain boundary in the superconductive element. To obtain an acceptable control the substrate is dimpled to a thickness of 40-50 .mu.m. To operate this device still voltages around 80 V are needed. On the other hand a thinner substrate would lead to stability problems.
All in all High-T.sub.c FET devices have not yet evolved into viable devices, although many of them have been suggested. A figure of merit for a JOFET is its superconducting current I.sub.S vs. drain-source-voltage U.sub.ds characteristic which should have a high critical or maximum superconducting current I.sub.C and a high resistivity above the critical current I.sub.C. In the following this is called a good superconducting behavior.